This invention relates to filters which, although not limited thereto, are particularly useful in sampling devices for detecting particulate contaminants in a gaseous medium, particularly radioactive contaminants.
Various filters are known in the prior art but have various disadvantages which are alleviated by the present invention.
U.S. Pat. No. 3,109,096 of Spaa issued Oct. 29, 1963, discloses a system for determining the radioactivity of gas sols (airborne particles and gaseous aerosols) which utilizes a continuous roll of filter paper. However, it is difficult to obtain a gas-tight seal with the filter paper at the detector station and the filter paper can be easily damaged. Rolls of filter paper are also somewhat awkward to handle.
U.S. Pat. No. 3,555,278 of Schroeder, issued Jan. 12, 1971, discloses apparatus for measuring potential alpha ray activity of a gas sample containing radon which produces daughter products, some of whom are alpha ray emitters. The apparatus uses filter paper which is held in place by tape or by clamping. It is inconvenient to handle the filter paper in this manner and, if care is not taken, it may easily be damaged.
U.S. Pat. No. 3,654,801 of Keefer et al, issued Apr. 11, 1972, also discloses apparatus using a continuous tape of filter material which passes from a supply roll to a take up roll. The disadvantages are the same as mentioned above in connection with U.S. Pat. No. 3,109,096. Similar comments apply to the arrangement disclosed in U.S. Pat. No. 4,144,454 of Groh et al, issued Mar. 13, 1979.